According to statistics released by the United States Fire Administration National Fire Data Center (https://www.usfa.fema.gov/downloads/pdf/statistics/v14i13.pdf), electrical arcing in residential electrical wiring account for over 70% of electrical fires—which are one of the most dangerous threats to life and property. Arc faults (also called arcs) are high-power, continuous electric discharges between two or more conductors—typically occurring in residential buildings when the integrity of an electrical wire or insulation is compromised (e.g., through physical damage, water damage, corrosion, age, or loose connections, among others). Events such as lightning strikes and power surges can also initiate the breakdown of insulation and lead to a compromised wire. As a result of the compromised wire, small, sporadic electrical discharges begin to occur and the insulating material that surrounds the wire is carbonized. As the electrical discharges continue over time, the insulation is increasingly eroded, and the electrical discharges increase in intensity. Eventually, strong electrical discharges become continuous arc faults that form in the wire—resulting in large flow of current and large releases of energy (with correspondingly high temperatures). Due to the proximity of the wire to wood frame, insulation, and/or similar combustible materials, when the temperatures produced by the arcs are high enough, they are likely to produce fire. It would be a great advantage in preventing electrical fires if one could detect, and be warned about, the small electrical discharges that may occur for days, weeks, or months before they become large enough to create electrical arcing (as is described in Yereance, R. A., and Kerkhoff, T., Electrical Fire Analysis, 3rd ed., page 206, Charles C. Thomas, Springfield Ill. (2010)).
The above-mentioned electrical discharges can occur in various ways, including: parallel, series, and line-to-ground discharges. A parallel electrical discharge occurs when current/electrons flows from one conductor to another through a gas or dielectric material because of a large voltage difference between the conductors—typically through damaged insulation or the air. FIG. 1A is a diagram of a parallel electrical discharge. The electrical circuit has two wires 102a, 102b, each of which is surrounded by an insulating material. If the insulation between the wires breaks down, then electrical discharges (such as discharge 104) can occur between the wires. Examples of parallel electrical discharges include carbonization (i.e., breakdown of the insulating material) and wet tracking (i.e., moisture on the surface of the wire that enables currents to form).
A series electrical discharge occurs when a single conductor is damaged to an extent that resistance through the conductor is increased and creates voltage differences high enough for discharges to occur within the conductor and into the surrounding insulation (or even to external objects, if the conductor is exposed). FIG. 1B is a diagram of a series electrical discharge. The electrical circuit includes a damaged wire 106 that produces an electrical discharge 108. Examples of series electrical discharges include ground pyrolysis (i.e., current flowing from the conductor to nearby wood), and last strand (i.e., breakage of the wire resulting in increase in heat and ignitable gases). A special type of series discharges occurs in a phenomenon known as a glowing connection (as shown in FIG. 1C). In such cases electrical conductors are touching, but not firmly connected together. An oxide layer forms at the boundary of the interface which increases the resistance of the conductors at the junction. If current is flowing through the interface, the temperature can rise to dangerous levels (e.g., the white area 110 shows a high temperature at an electrical outlet) which can ignite nearby materials and cause a destructive fire. By the time an electrical discharge has progressed to the point that it causes a fire, it is too late to take corrective action and prevent loss. It is important to detect the occurrence of such electrical discharges in electrical wiring as early as possible so that remedial measures can be put in place.
Technology such as arc-fault circuit interrupters (AFCI) currently exist to detect early-stage electrical discharges, such as arc faults. In electrical outlets equipped with AFCI technology, an AFCI detects arc faults in a circuit and breaks the circuit upon detection of such faults to prevent an electrical fire from happening. However, AFCIs are relatively expensive and must be installed on each circuit in a building to detect electrical discharges on the individual circuits.